This invention relates to magnetic recording media such as magnetic recording tapes, floppy disks, magnetic recording disks, etc., a process for producing the same, and magnetic memory apparatuses using the magnetic recording media, and more particularly to magnetic recording media for longitudinal recording suitable for high density magnetic recording, a process for producing the same and magnetic memory apparatuses.
Heretofore, magnetic recording media using a metallic magnetic film have been proposed as magnetic recording-media for longitudinal recording for high density magnetic recording, as disclosed in Japanese Patent Publication No. 54-33523. Processes for forming magnetic recording media for longitudinal recording include an evaporation process, a sputtering process, a plating process, an ion beam sputtering process, etc.
Recently, needs for higher density recording and higher reliability have been increased. For example, magnetic recording media for longitudinal recording with a thin metallic magnetic film having an inplane coercivity as high as about 700 Oe and a high corrosion resistance at a high temperature and a high humidity, such as thin metallic magnetic film of magnetic alloy, e.g. Co-Pt, Co-Cr-Pt, Co-Ta-Pt, Co-Si-Pt, Co-Zr-Pt, Co-Hf-Pt, etc. have been proposed as in Japanese Patent Applications Kokai (Laid-open) Nos. 60-111323, 59-177725, 59-8806, etc.
Furthermore, it has been proposed to improve static magnetic properties such as an inplane coercivity Hc, a squareness S, a coercive squareness S*, etc. For example, it has been proposed to form a pure metal layer of Cr, Mo, W, Nh, W, etc. or an alloy layer of Cr-V, Cr-Fe, etc. as an underlayer on a substrate and form a magnetic recording layer of Co-Pt alloy or Co-Cr-Pt alloy thereon, an disclosed in Japanese Patent Application Kokai (Laid- open) No. 62-257617 (=U.S. Pat. No. 4,654,276), or to form a magnetic recording layer of Co-Cr-Pt alloy on a Ni-P underlayer, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 59-88806, or to form an alumite underlayer on a substrate of aluminum alloy and form a magnetic recording layer of cobalt (Co)-based alloy comprising 3 to 15 at.% of at least Mo, V and W, 3 to 20 at.% of Cr and 3 to 15 at.% of a noble metal element such as Pt. Rh, Ru, Re, Pd, Ir, etc., the balance being at least 75 at.% of Co, thereon, as disclosed in Japanese Patent Applications Kokai (Laid-open) Nos. 61-246917 and 61-253622.
Furthermore, Japanese Patent Application Kokai (Laid-open) No. 62-257617 (=U.S. Pat. No. 4,654,276) and Japanese Patent Application Kokai (Laid-open) No. 62-257618 (=U.S. Pat. No. 4,652,499) disclose that when a magnetic recording layer of Co-Pt or Co-Cr-Pt is produced on a non-magnetic underlayer having a thickness of about 50 nm, such as Cr-V underlayer or W underlayer, the inplane coercivity Hc can be made higher than 1,200 Oe and also the coercive squareness S* higher than 0.9.
The main object of the present invention is to improve the corrosion resistance of a metallic magnetic layer and also to increase the inplane coercivity and the coercive squareness, thereby obtaining a higher density magnetic recording and a higher read output. Magnetic recording media of the prior art generally have such a disadvantage that the media noise tends to increase with a higher density magnetic recording and a higher read output. Particularly with a recent higher density magnetic recording, the recording frequency has been increased and the band width has been broadened, and consequently the head noise and amplifier noise tend to increase. Thus, it has been desired to develop magnetic recording media having smaller noise characteristics than these noises, while maintaining a higher read output.
As the result of extensive studies, the present inventors have found that an increase in the inplane coercivity and coercive squareness can increase the read and write characteristics at a higher density, but also can increase the noise and thus is not always advantageous with respect to the signal-to-noise ratio, and that particularly the noise considerably increases when the coercive squareness is made more than 0.9. Thus, in order to obtain magnetic recording media for longitudinal recording with distinguished read and write characteristics, it is essential to satisfy these mutually contradicting magnetic properties at the same time, and it is a current task to satisfy an inplane coercivity Hc of not less than 1,200 Oe and a coercive squareness S* of not more than 0.9, preferably not more than 0.85 at the same time. A coercive squareness S* means a ratio of H to Hc (H/Hc) at a cross point of a tangent line drawn at the point of inplane coercivity Hc in a magnetic hysteresis loop with a straight line drawn at the point of remanence magnetization Mr and in parallel to the magnetic field (H) axis.
A first object of the present invention is to provide magnetic recording media for longitudinal recording with less noises, a distinguished S/N ratio and a high reliability in corrosion resistance, etc.
A second object of the present invention is to provide magnetic recording media for longitudinal recording with a high inplane coercivity Hc, that is, at least 1,200 Oe and a small coercive squareness S*, that is, not more than 0.9, preferably not more than 0.85, which can read and write in a high S/N ratio even at a high density recording and has a high reliability, that is, high corrosion resistance and antiwear properties.
A third object of the present invention is to provide a process for producing magnetic recording media for longitudinal recording that can attain the second object of the present invention.
A fourth object of the present invention is to provide magnetic memory apparatuses using the magnetic recording media for longitudinal recording that can attain the first or second object or both objects of the present invention.
The first object of the present invention can be attained by making a magnetic layer mainly from an alloy comprising Co, a material X composed of at least one element selected from the first group consisting of Cr, Mo and W, a material Yxe2x80x2 composed of at least one element selected from the second group consisting of Ti, Zr, Hf, Ta, Nb, Ru and Rh, and a material Z composed of at least one element selected from the third group consisting of Al and Si. The alloy of the magnetic layer is represented by the following general formula:
(Co1-aXa)1-b-cYxe2x80x2bZc, 
wherein it is desirable that a concentration of X on the basis of Co, that is, 100a, is 3 at.% to 20 at.% and concentrations of Yxe2x80x2 and Z on the basis of the sum total of Co and X, that is, 100b and 10c, are 1 at.% to 15 at.% and 1 at.% to 15 at.%, respectively, where the concentration of inevitable impurities is disregarded. Furthermore, it is desirable that the magnetic layer contains 0.1 at.% to 15 at.% of oxygen.
Furthermore, it is particularly desirable with respect to an improvement of inplane coercivity to provide an intermediate layer of nonmagnetic material composed mainly of at least one of Cr, Mo and W, and their alloys such as Cr-Ti, etc. between the magnetic layer and the nonmagnetic substrate. With magnetic recording media of the foregoing structure, magnetic memory apparatuses of high capacity with a high reliability can be provided.
The effects of the magnetic layer of the foregoing structure can be obtained through the following functions. The functions of the present invention will be explained below, referring to use of a body centered cubic (bcc) metal such as alloys composed mainly of at least one of Cr, Mo and W and their alloys such as Cr-Ti, etc. as an underlayer. On the underlayer, the axis of magnetic anisotropy of Co is oriented to have an inplane anisotropic component so as to give a high inplane coercivity. Furthermore, by addition of at least 3 at.% of Cr, etc. to Co in the magnetic layer, a high inplane coercivity, for example, about 500 Oe or higher, can be obtained. With increasing Cr concentration, the corrosion resistance of Co alloy increases, whereas the noise of Co alloy decreases. However, the saturation magnetization is abruptly deteriorated with increasing Cr concentration, and thus more than 20 at.% of Cr to be added is not preferable.
In order to improve the saturation magnetization, the present inventors have made extensive studies of additive elements and have found that addition of Ti, Zr, Hf, V, Nb, Ta, Fe, Ru, Os, Rh, Ir, Pd, La, Sm, Pr, etc. can increase the saturation magnetization, but the media noise at the read and write runs is large with these additive elements. That is, the saturation magnetization could be improved, whereas there appeared a new problem of an increase in the media noise. Thus, in order to reduce the media noise, the present inventors have also made extensive studies of other additive elements, and have found that the media noise can be reduced by adding at least one element selected from the group consisting of Ti, Zr, Hf, Nb, Ta, Ru and Rh, that is, the aforementioned second group, used as additive elements for the improvement of the saturation magnetization and also by adding at least one element selected from-the group consisting of Al and Si, that is, the aforementioned third group, thereto. Neither deterioration of corrosion resistance nor reduction in the saturation magnetization has been observed at all in that case.
In case of magnetic layers using materials X other than Cr, selected from the aforementioned first group, the same function and effects as in case of Cr can be obtained.
The reduction in the media noise seems mainly due to the fact that an alloy of material Yxe2x80x2 selected from the second group and material Z selected from the third group segragates at the magnetic crystalline grain boundary,. That is, the alloy is nonmagnetic and thus the magnetic interaction (exchange coupling, magnetstatic coupling, etc.) is reduced among the magnetic crystalline grains. As a result, the number of crystalline grains constituting the minimum unit region (cluster) of magnetization reversal is decreased, as compared with that of the conventional crystalline grains. (A possible minimum cluster is composed of a simple crystal grain.) Thus, the width of magnetic transition region as a cause for media noise is narrowed, resulting in reduction of media noise. Particularly when a magnetic layer is formed so as to contain 0.1 at.% to 15 at.% of oxygen, the magnetic crystalline grains are made finer and the magnetic grain boundaries are made thicker and/or denser to reduce the magnetic interaction much more. Thus, the media noise is further reduced and the corrosion resistance is also improved.
The desirable concentrations of the additive elements can be explained below. FIG. 2 shows media noise characteristics of magnetic recording disks prepared in the following manner, when subjected to read and write runs with a Mn-Zn ferrite ring head with a gap length of 0.6 xcexcm. The magnetic disks were prepared by forming a Cr underlayer having a thickness of 350 nm on an Al-Mg alloy substrate plated with Ni-P, 130 mm in diameter, a magnetic layer of (Co0.90Cr0.10)0.96-cTa0.04Alc having a thickness of 70 nm and a C (carbon) protective layer having a thickness of 40 nm successively thereon by RF magnetron sputtering at a substrate temperature of 1000xc2x0 C. under an argon gas pressure of 15 mTorr with an input power density of 1 W/cm2. The relative head-to media speed is 15 m/sec and the recording frequency is 7 MHz. As shown in FIG. 2, the media noise is abruptly reduced with increasing Al concentration in the magnetic layer, and its reduction is saturated around 10 at.% of Al. Thus, even addition of more than 15 at.% of Al is less effective for the reduction of media noise. Furthermore, the saturation magnetization and inplane coercivity are decreased with increasing Al concentration, resulting in a decrease in the read output.
On the other hand, reduction in the total noise NT of magnetic recording system is only about 20%, when a MnZn ferrite ring head or a metal-in-gap type lead (MIG head) is used as a head and even when the media noise is reduced to 4 xcexcVrms from 7 xcexcVrms. Thus, addition of at least 1 at.% of Al is satisfactory and addition of 2.5 at.% or more of Al is more preferable for remarkable reduction of the media noise. Similar effects are obtained in case of addition of Si or Al-Si alloy in place of Al.
The reason why there is no significant difference in the total noise of a magnetic recording system between the media noise of 7 xcexcVrms and that of 4 xcexcVrms is explained as follows:
The total noise NT of the system is a function of media noise N and a head amplifier noise NHA and can be represented by the following equation (1):                               N          T                =                                            N              2                        +                          N              HA              2                                                          (        1        )            
When a MnZn ferrite ring head-or a metal-in-gap type head is used as a head, the head amphifier noise NHA becomes a constant-of about 7 xcexcVrms.
Thus, the equation (1) will be as follows:       N    T    =                    N        2            +              7        2            
Thus, when N is decreased to 4 xcexcVrms from 7 xcexcVrms, NT will be reduced by about 20%.
Corrosion resistance of
C/(Co0.89Cr0.11)0.95-bZrbSi0.05/Cr media 
prepared under the same conditions as above was evaluated by a temperature/humidity corrosion test at 70xc2x0 C. and 85% RH, and it was found that at 1 at.% or more of Zr no read and write error was observable even after 3 weeks, and a good corrosion resistance could-be obtained, whereas above 15 at.% of Zr, deterioration of saturation magnetization and inplane coercivity was remarkable and no high read output was obtained. At 1 to 15 at.% of Zr, Hc was high, i.e. 700 Oe or more. Similar effects were obtained with Ti, Hf, Nb, Ta, Ru and Rh or their alloys in place of Zr.
By allowing an Ar gas to contain 0.02 vol.% to 1.0 vol.% of oxygen when the magnetic layer of foregoing composition is formed, the magnetic layer can contain 0.1 at.% to 15 at.% of oxygen. In that case, the magnetic crystalline grains are made finer with increasing concentration of oxygen and also the oxide segragates at the grain boundary, resulting in reduction of the aforementioned magnetic interaction and improvement of the corrosion resistance.
It is needless to say that a magnetic memory apparatus of larger memory capacity with a high reliability can be provided when a magnetic recording disk, a floppy disk or a magnetic recording tape of the foregoing structure is used.
Furthermore,. the first object of the present invention can be also attained by making a magnetic layer mainly from an alloy containing Co and Pt and further containing a material X composed of at least one element selected from the first group consisting of Ni, Cr, Mo and W, a material Yxe2x80x2 composed of at least one element selected from the second group consisting of Ti, Zr, Hf, Ta, Nb, Ru and Rh and a material Z composed of at least one element selected from the third group consisting of Al and Si.
The alloy of the magnetic layer can be represented by any one of the following general formulae:
(Co1-aNia)1-b-c-dYxe2x80x2bZcPtd, 
(Co1-axe2x80x2Bxe2x80x2axe2x80x2)1-b-c-dYxe2x80x2bZcPtd, or 
(Co1-a-eNiaBxe2x80x2e)1-b-c-dYxe2x80x2bZcPtd, 
where it is desirable that a concentration, 100d, of Pt is 0.1 at.% to 30 at.%, a concentration, 100c, of material Z is 1 at.% to 15 at.%, and a concentration, 1Ob, of material Yxe2x80x2 is 1 at.% to 15 at.%, the balance being Co and X. Bxe2x80x2 is a material composed of at least one element selected from the group consisting of Cr, Mo and W. It is desirable that a is 0.1 to 0.5, axe2x80x2 is 0.01 to 0.2 and e is 0.01 to 0.15, where the concentration of inevitable impurity is disregarded. It is further desirable with respect to the reduction of media noise that the magnetic layer contains 0.1 at.% to 15 at.% of oxygen, and it is also preferable with respect of an improvement of inplane coercivity to provide an intermediate layer of non-magnetic material composed mainly of at least one of Cr, Mo and W, and their alloys such as Cr-Ti, etc. between the magnetic layer and the nonmagnetic substrate. With magnetic recording media of the foregoing structure, magnetic memory apparatuses of high reliability suitable for a high density magnetic recording can be provided.
The effects of the magnetic layer of the foregoing structure can be obtained through the following functions. The functions of the present invention will be explained below, referring to use of a body centered cubic (bcc) metal such as alloys composed mainly of at least one of Cr, Mo and W and their alloys such as Cr-Ti, etc. as an underlayer. On the underlayer, the Co layer is grown so that the axis of magnetic anisotropy of Co is oriented to have an inplane anisotropic component and thus to give a high inplane coercivity. Furthermore, by addition of Ni, Cr, Mo, W, Pt, etc. to Co in the magnetic layer, a higher inplane coercivity, for example, about 500 Oe or higher, can be obtained.
As a result of further studies, the present inventors have found that by addition of Pt and at least one element selected from the first group consisting of Ni, Cr, Mo and W to Co, a higher inplane coercivity, an appropriately high saturation magnetization and a high read output can be obtained, but that these alloy still have problems of large media noise and poor corrosion resistance. Thus, the present inventors have further studied addition of various elements of groups 4a, 5a, 6a, 8, 8b and 4b of the periodic table to the alloys composed of Co, Pt and the material selected from the first group, such as CoNiPt, CoCrPt, CoNiCrPt, etc. to reduce the media noise and improve the corrosion resistance while maintaining a high inplane coercivity, and have found that the material composed of at least one element selected from the third group consisting of Al and Si, as added, segregates at the magnetic crystalline boundary of a magnetic layer containing Co as the main component to reduce the magnetic interaction among the magnetic crystalline grains, thereby considerably reducing-the media noise. However, it has been found that the corrosion resistance is not improved, but rather deteriorated, because it seems that the segregates at the magnetic crystalline grain boundary cause to more easily form local cells between the magnetic crystalline grains and the grain boundary. In order to solve this problem, the present inventors have studied further addition of elements of groups 4a, 5a, 6a, 8, 3b, 4b, etc. of the periodic table. As a result of evaluation of the corrosion resistance by a NaCl spray test, it has been found that the corrosion resistance can be considerably improved without any deterioration of media noise by further addition of a material composed of at least one element selected from the second group consisting of Ti, Zr, Hf, Ta, Nb, Ru and Rh, because elements of Ti, Zr, Hf, Ta, Nb, etc. form a dense passivation film at the grain boundary or elements of Ru, Rh, etc. make the oxidation-reduction potential of the magnetic crystalline grains nobler without the increase of magnetic interactions among the grains. It has been further found that the highest corrosion resistance can be obtained without much deterioration of media noise by addition of an alloy composed of elements selected from these two groups to give these two effects. As-far as the corrosion resistance is concerned, the corrosion resistance can be improved by adding Pd, Pr, etc., for example, to a CoNiPtSi-based alloy, but the media noise is increased thereby.
The concentrations of the aforementioned additive elements will be explained below. FIG. 3 shows a dependence of inplane coercivity on Ni concentration of magnetic recording disks prepared in the following manner. A Cr underlayer having a thickness of 420 nm is formed on an Al-Mg alloy substrate plated with Ni-P, {fraction (51/41+L )} in diameter, and a magnetic layer of (Co1-aNia)0.85Pt0.05Si0.05Zr0.05 having a thickness of 60 nm is formed thereon by RF magnetic sputtering at a substrate temperature of 1000xc2x0 C. under an argon gas pressure of 15 mTorr with an input power density of 1.5 W/cm2. It is obvious from FIG. 3 that at a Ni concentration 100a of 10 at.% to 60 at.% on the basis of Co, a high inplane coercivity, that is, 700 oe or higher, can be obtained.
In case of magnetic recording media of (Co1-axe2x80x2Craxe2x80x2)0.85Pt0.05Si0.05Ta0.05/Cr prepared in the same manner as above, a high inplane coercivity, that is, 700 Oe or higher, can be obtained at a Cr concentration 100axe2x80x2 of 3 at.% to 20 at.% on the basis of Co.
In case of magnetic recording media of (co1-a-eNiaCre)0.85Pt0.05Al0.05Zr0.05/Cr prepared under the same conditions as above, a high inplance coercivity, that is, 700 Oe or higher, can be obtained in ranges of 0.1 less than a less than 0.5 and 0.01 less than e less than 0.15.
The same effects can be also obtained with Mo or W in place of Cr or with Ti, Hf, Nb, Ru, Rh or their alloys in place of Ta and Zr as additive elements to the magnetic layer.
FIG. 4 shows a dependence of the inplane coercivity of magnetic recording media of (Co0.7Ni0.3)0.9-dPtdAl0.05Zr0.05/Cr prepared under the same conditions as above upon a Pt concentration d. A particularly high inplane coercivity, that is, 1,000 Oe or higher, can be obtained with a high read output at a Pt concentration 100d of 0.1 at.% to 20 at.%, preferably 2 at.% to 10 at.%.
FIG. 5 shows a result of evaluation of read and write characteristics of magnetic recording disks comprising a magnetic layer of (Co0.7Ni0.3)0.9-cZr0.05SicPt0.05 having a thickness of 60 nm, a carbon (C) protective layer having a thickness of 40 nm formed thereon under the same conditions as above, and a layer of highly fluorinated liquid lubricant of perfluoroalkylpolyether by means of a Mn-Zn ferrite ring head. As shown in FIG. 5, the media noise is abruptly decreased with increasing Si concentration, and its decrease is saturated around 10 at.% of Si.
Thus, addition of more than 15 at.% of Si is less effective for the reduction of media noise. Furthermore, the saturation magnetic flux density and the inplane coercivity are abruptly decreased with increasing Si concentration,, resulting in a decrease in the read output.
On the other hand it is obvious therefrom that the effect on the reduction of media noise is remarkable at a Si concentration of 1 at.% or higher, preferably 3 at.% or higher in the same manner as above. The same effect is also observable in case of adding Al or an alloy of Al and Si.
Magnetic recording media of C/(Co0.6Ni0.4)0.9-bZrbAl0.05Pt0.05/Cr were prepared under the same conditions as above, and their corrosion resistance was evaluated by a NaCl spray test. It was found that the corrosion resistance could be improved 5-fold or more without any deterioration of media noise by selecting a Zr concentration 100b of 1 at.%.or more. Addition of more than 15 at.% of Zr was not desirable because the saturation magnetic flux density and the inplane -coercivity were considerably lowered, resulting in a decrease in the read output. An inplane coercivity of 700 Oe or more could be obtained at a Zr concentration of 1 at.% to 15 at.%. The same effect could be also obtained with Ti, Hf, Ta, Nb, Ru, Rh and their alloys in place of Zr.
By allowing the Ar gas to contain 0.05 vol.% to 2 vol.% of oxygen when a magnetic layer is formed from the magnetic materials of the foregoing composition, the magnetic layer can contain 0.1at.% to 15 at.% of oxygen, and the magnetic crystalline grains can be made fine with increasing concentration of oxygen and the oxide segregates at the grain boundary to reduce the magnetic interaction among the magnetic crystalline grains, thereby further reducing the media noise. At the same time, the strength of film surface passivation layer can be increased at the same time and thus the corrosion resistance can be improved.
With magnetic recording disks, floppy disks or magnetic recording tapes of the foregoing structure in combination with a magnetic head such as a MnZn ferrite ring head or a metal-in-gap type head, a magnetic memory apparatus with distinguished read and write characteristics and a high reliability can be provided.
The second object of the present invention can be attained by magnetic recording media for longitudinal recording, which comprises a nonmagnetic substrate, a nonmagnetic metallic underlayer comprising at least one metal element selected from the group consisting of Cr, Mo, W, V, Nb and Ta formed on the nonmagnetic substrate and a magnetic layer of Co-based alloy formed on the nonmagnetic metallic underlayer, the Co-based alloy comprising 1 to 35 at.% of at least one first additive element selected from the group consisting of Pt and Ir, 1 to 17 at.%, preferably 3 to 15 at.%, of at least one second additive element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ge and Si, except for Si, whose concentration is 1 to,40 at.%, preferably 2 to 30 at.%, and 0.1 to 10 at.% of oxygen, sum total of the first and second additive element and oxygen being 2.2 to 50 at.%, the balance being Co. As shown in FIG. 8, the thickness of the nonmagnetic metal underlayer is preferably 150 nm or more, and more preferably 200 nm or more with respect to an improvement of inplane coercivity. When the magnetic layer is formed on the metallic underlayer, the inplane components of crystalline orientation of the magnetic layer can be increased, which can improve the inplane coercivity.
When the thickness of the nonmagnetic metallic underlayer exceeds 600 nm, a problem of surface roughness is serious, and the flyability of a magnetic head is deteriorated. A larger thickness also results in a higher cost. Thus, it is desirable that the film thickness of a nonmagnetic metallic underlayer is not more than 600 nm. More preferable concentrations of the magnetic layer of Co-based alloy are 3 to 13 at.%, preferably 5 to 9 at.%, of the first additive element and 3 to 15 at.% of the second additive element, except for Si, whose concentration is 2 to 30 at.%.
The second additive elements will be further explained below: It is particularly preferable to select Cr, Mo, W, Ge and Si from the group and it is also desirable that at least one of these elements is contained as an essential additive component. That is, when the second additive elements are classified into group A consisting of Cr, Mo, W, Ge and Si and group B consisting of Ti, Zr, Hf, V, Nb and Ta, at least one element selected from the group A and at least one element selected from the group B must be contained at the same time, or at least one element selected from the group A must be contained as an essential component. A preferable concentration of the additive element from the group A is 3 to 15 at.%, as described above, except for Si, whose concentration is 2 to 30 at.%, and a preferable concentration of the additive element from the group B is 1 to 15 at.%.
The nonmagnetic metallic underlayer can be composed of at least one metal element selected from the group consisting of Cr, Mo, W, V, Nb and Ta and at least one element selected from the group consisting-of Ti, Si, Ge, Cu, Pt, Rh, Ru, Re, Pd and oxygen, where a preferable concentration of at least one element selected from the group consisting of Ti, Si, Ge and Cu-is l to 30 at.%, a preferable concentration of at least one element selected from the group consisting of Pt, Rh, Ru, Re and Pd is 0.01 to 10 at.% and a preferable concentration of oxygen is 0.1 to 10 at.%.
Such characteristics as an inplane coercive sequareness S* of not more than 0.95, preferably 0.85 to 0.4, more preferably 0.81 to 0.6 and an inplane coercivity Hc of not less than 1,200 Oe, preferably at least 1,500 Oe, can be obtained thereby, and a magnetic layer with distinguished corrosion resistance and S/N ratio can be obtained. Furthermore, addition of an-appropriate amount of Ni, Al, etc. to the magnetic layer of Co-based alloy -can improve the S/N ratio, though the corrosion resistance is deteriorated.
The third object of the present invention can be attained by a process for producing magnetic recording media for longitudinal recording, which comprises a step of forming a nonmagnetic metallic underlayer comprising at least one metal element selected from the group consisting of Cr, Mo, W, V, Nb and Ta on a nonmagnetic substrate by physical vapor deposition, a step of forming on the underlayer a magnetic layer of Co-based alloy comprising 1 to 35 at.% of at least one first additive element selected from the group consisting of Pt and Ir, 1 to 17 at.%, preferably 3 to 15 at.%, of at least one second additive element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ge-and Si, except for Si, whose concentration is 1 to 40 at.%, preferably 2 to 3O at.%, and 0.1 to 10 at.% of oxygen, total of the first and second additive elements and oxygen being 2.2 to 50 at.%, the balance being Co, in an inert gas containing a very small amount of oxygen by sputtering of a target containing the first and second additive elements, and a step of forming on the magnetic layer a protective layer. It is preferable to provide a step of forming an underlayer of nonmagnetic plating film such as a Ni-P plating film, etc. in advance to the step of forming the nonmagnetic metallic underlayer. The nonmagnetic metallic underlayer and the magnetic layer of Co-based alloy can be formed by sputtering, vapor deposition, plating, ion beam deposition, etc. Particularly, DC magnetron sputtering is preferable in respect to the deposition rate and layer quality control.
The nonmagnetic metallic underlayer comprising at least one metal element selected from the group consisting of Cr, Mo, W, V, Nb and Ta to be formed on the nonmagnetic substrate by physical vapor deposition can contain at least one element selected from the group consisting of Ti, Si, Ge, Cu, Pt, Rh, Ru, Re, Pd and oxygen as a secondary component. In that case, a preferable concentration of at least one element selected from the group consisting of Ti, Si, Ge and Cu is 1 to 30 at.%, a preferable concentration of at least one element selected from the group consisting of Pt, Rh, Ru, Re and Pd is 0.01 to 10 at.%, and a preferable concentration of oxygen is 0.1 to 10 at.%.
Furthermore, it is preferable to form a magnetic layer of Co-based alloy containing 3 to 13 at.%, preferably 5 to 9 at.%, of the first additive element and 3 to 15 at.% of the second additive element, except for Si, whose concentration is 2 to-30 at.%.
The second additive element will be further explained below. It is particularly preferable to select Cr, Mo, W, Ge and Si from the group and it is desirable to contain at least one of these elements as an essential component. That is, when the second additive elements are classified into group A consisting of Cr, Mo, W, Ge and Si and group B consisting of Ti, Zr, Hf, V, Nb and Ta, at least one element selected from the group A and at least one element selected from the group B must be contained at the same time, or at least one element selected from the group A must be contained as an essential component. A preferable concentration of the element of group A is 3 to 15 at.% except for Si, whose concentration is 2,to 30 at.%, and a preferable concentration of the element of group B is 1 to 15 at.%, as mentioned before. A magnetic layer must be formed under sputtering conditions that can satisfy the foregoing concentrations.
It is desirable to form a magnetic layer by sputtering, while maintaining the underlayer substrate in a heated state, which is practically preferably at 1000xc2x0 to 350xc2x0 C. Above 350xc2x0 C, the underlayer substrate will react with the magnetic layer, whereas at a temperature below 1000xc2x0 C. intermetallic compounds are easy to form, and the coercive squareness becomes abnormally larger. It is preferable to chemically and/or physically roughen the surface of a substrate (grooves, irregularities or scars) by an etching and/or abrasing treatment, for example, generally called xe2x80x9ctexturing treatmentxe2x80x9d in advance as a preliminary step for forming a magnetic layer. For example, by making fine grooves, irregularities or scars, 2 nm to 30 nm in terms of centerline average roughness on a disk substrate in the moving direction of a magnetic head, the crystalline grains of the nonmagnetic metallic underlayer on the disk substrate surface and the magnetic layer formed thereon undergo crystalline orientation particularly in the moving direction of the magnetic head, whereby magnetic properties such as squareness, inplane coercivity, etc. in the moving direction of the magnetic head can be considerably improved. The texturing treatment can contribute to an improvement of magnetic properties in accordance with the substrate heating when a magnetic layer is formed. Particularly, the disks with surface irregularities and/or scars are preferable to show much improved CSS (contact-start/stop) characteristic.
The reasons why the second and third objects of the present invention can be attained will be explained in detail below:
The nonmagnetic metallic underlayer of pure metal or an alloy comprising at least one metal element selected from the group consisting of Cr, Mo, W, V, Nb and Ta, provided on the nonmagnetic substrate, gives a large influence on the crystallographic orientation and the magnetic properties of a magnetic layer of Co-based alloy to be formed on the surface of the nonmagnetic metallic underlayer. That is, the above-mentioned nonmagnetic metallic underlayer has a body-centered cubic structure and is liable to have a (110) orientation on the nonmagnetic substrate. The magnetic layer to be formed thereon easily undergoes epitaxial growth and thus has more inplane components of magnetic anisotropy. Thus, the nonmagnetic metallic underlayer acts to increase the inplane coercivity Hc of the magnetic layer.
FIG. 13A and FIG. 13B show X-ray diffraction patterns of magnetic recording media for longitudinal recording and the crystallographic orientation and the crystallinity of magnetic layers and underlayers according to embodiments of the present invention, respectively. That is, a CrTi alloy underlayer with various Ti concentrations of 1 to 30 at.% having a thickness of 10 to 500 nm, a magnetic layer of Co-15 at.% Cr-8 at.% Pt-l at.% Si having a thickness of 50 nm, and a C protective layer having a thickness of 30 nm were formed on a strengthened glass substrate, 3.5xe2x80x3 in diameter, in succession by DC sputtering in a Ar gas atmosphere containing 0.1 vol.% of oxygen at a substrate temperature of 110xc2x0 C under a gas pressure of 10 mTorr with an input power density of 1 W/cm2.
As shown in FIG. 13B, CrTi mainly takes a (100) orientation when the CrTi underlayer has a thickness of less than 0.05 xcexcm, and the component of (110) orientation will abruptly increase when the CrTi underlayer thickness exceeds 0.15 xcexcm (150 nm) and the CoCrPtSi magnetic layer will also epitaxially grow and take a (10{overscore (1)}1) orientation with the inplane c-axis component. Here, c-axis is the principal axis of the magnetic anisotropy of CoCrPtSi. By providing a CrTi underlayer, the (10{overscore (1)}0) orientation, where the c-axis exists in the plane, will also develop at the same time. Thus, by providing a CrTi underlayer, the (10{overscore (1)}1) and (10{overscore (1)}0) orientations, where the c-axis of CoCrPtSi has inplane components, are developed, resulting in a higher inplane coercivity. In that case, the magnetic layer has an oxygen concentration of 2 at.%.
Relationships shown in FIGS. 13A and 13B are not limited to CoCrPtSi and CrTi, and also hold valid in other embodiments of the present invention.
The thickness of the nonmagnetic metallic underlayer also plays an important role to control not only the inplane coercivity Hc, but also the coercive squareness S*, as will be explained,.referring to FIG. 8.
FIG. 8 is a diagram of characteristic curve showing relationships between the thickness of nonmagnetic metallic underlayer and the magnetic properties Hc and S* of magnetic recording media prepared by forming an Ni-P plating layer on a nonmagnetic Al-Mg alloy disk substrate by a known method, and then forming a nonmagnetic metallic underlayer (exemplified by Cr in this case) and a magnetic layer (exemplified by Co80Cr10Pt10 in this case) of the present invention thereon in succession at a substrate temperature of 150xc2x0 C., where the thickness of the magnetic layer of the magnetic recording media is made constant to 75 nm. Here 2 at.% of oxygen is contained in the magnetic layer.
As is obvious from FIG. 8, a remarkable abrupt change is observable around an underlayer thickness of 150 nm, that is, the inplane coercivity Hc exceeds 1,200 Oe in the region where the inplane coercive squareness S* is lower than 0.85, and when the underlayer thickness exceeds 200 nm, S* becomes less than 0.8 and Hc becomes exceeds 1,500 Oe, resulting in a higher recording density and in a higher S/N ratio. That is, an increased underlayer thickness can improve the crystalline orientation of underlayer and thus can drastically improve the magnetic properties. In this manner, the overall strength of the layers in combination is improved and thus the antiwear properties are also improved. However, when the underlayer thickness exceeds 600 nm, the nonmagnetic metal that forms the underlayer is liable to grow abnormally and the roughness of magnetic layer surface is increased. That is, the magnetic layer surface becomes coarser and the magnetic head flyability will be deteriorated. The production cost will also be higher in this case. Thus, it is desirable that the underlayer thickness is not more than 600 nm. That is, a practically preferable thickness of nonmagnetic metallic underlayer is 150 to 600 nm, more preferably 200 to 450 nm.
Relationships between the inplane coercive squareness S* and the media noise of magnetic recording media are shown by a characteristic curve in FIG. 11. When S* exceeds 0.85, the media noise abruptly increases, whereas when S* becomes less than 0.4, the read output waveform will be deformed. Thus, a practical S* is 0.85 to 0.4, preferably 0.81 to 0.5, more preferably 0.75 to 0.6.
When the nonmagnetic metallic underlayer of Cr, Mo, W, etc. contains 0.1 to 10 at.% of oxygen, the crystalline grains in the magnetic layer, which epitaxially grows thereon, will have grain sizes of not more than 100 nm,, resulting in a decrease in the media noise. Thus, this is particularly preferable. However, when the oxygen concentration-of the underlayer exceeds 10 at.%, the epitaxial growth is considerably suppressed and the inplane coercivity will be deteriorated.
Furthermore, at least one element selected from the group consisting of Ti, Si, Ge, Cu, Pt, Ru, Rh, Re and Pd to be contained in the nonmagnetic metallic underlayer can make the crystalline grains of the underlayer finer as in the case of oxygen as contained, and can also make the coercive squareness of a magnetic layer to be formed thereon not more than 0.85, resulting in a decrease in the media noise. In this case,- as shown in FIG. 13A and FIG. 13B, the crystalline orientation of the underlayer will be increased, and as shown in FIG. 15 the effect upon the inplane coercivity and the read output is also increased. Thus, this is particularly preferable. Practically preferable concentrations of these elements are 1 to 30 at.% for at least one element selected from the group consisting of Ti, Si, Ge and Cu and 0.01 to 10 at.% for at least one element selected from the group consisting of Pt, Ru, Rh, Re and Pd. A lower concentration will make the effect unsatisfactory, whereas a higher concentration will suppress the epitaxial growth, deteriorate the inplane coercivity or excessively increase the S*, resulting in deterioration of the read and write characteristics. Thus, a lower or higher concentration is not desirable.
It is particularly preferable to treat the substrate underlayer surface of Ni-P, etc. to have fine scars and/or grooves substantially in the head moving direction as a preliminary step before the formation of a nonmagnetic metallic underlayer to make a centerline average roughness Ra of 1 to 10 nm in the head moving direction and 2 to 30 nm in the direction perpendicular to the head moving direction, whereby the inplane coercivity can be made higher in the head moving direction than in the radial direction and also the read output can be made higher by one tenth to two tenths. Thus, this is particularly preferable and is due to the so called graphoepitoxial effect that a nonmagnetic metallic underlayer develops in accordance with the shape of the substrate underlayer, as clarified by observation with SEM, etc. When Ra in the radial direction is not more than 2 nm, the effect is smaller, whereas when it is more than 30 nm, the antiwear properties will be deteriorated.
Relationships between the concentrations of magnetic layer of Co-based alloy and the magnetic properties will be explained below.
The role of 1 to 35 at.% of at least one first additive element selected from the group consisting of Pt and Ir to be contained in a composition containing Co as the main component is principally to increase the inplane coercivity. However, this role can be played through an interaction in the presence of 1 to 17 at.%, preferably 3 to 15 at.% of at least one second additive element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ge and Sit except for Si, whose concentration is 1 to 40 at.%, preferably 2 to 30 at.%, and 0.1 to 10 at.% of oxygen, as in the case of the inplane corecive squareness S*, and the role is not played through the single action of the individual first additive elements for the following facts gathered by the present inventors from the following experimental results. A higher inplane coercivity can be easily obtained by adding only the first additive element (Pt and Ir) to Co, but it is difficult to stably obtain a high coercivity of 1,000 Oe or more only through such combinations as above, and furthermore the media noise is higher and a higher S/N ratio is hard to obtain. The present inventors have found that magnetic recording media with higher inplane coercivity Hc and crystalline orientation and a higher S/N ratio even at a high density recording can be obtained by adding the first additive element and the second additive element to Co and making a magnetic layer therefrom in a discharge gas atmosphere containing oxygen. When Co contains the second additive element and when the discharge gas atmosphere such as Ar, etc. contain an oxygen gas at the formation of a magnetic layer by sputtering, the second additive element segregates at the grain boundary and in the grains with the help of the oxygen to reduce the magnetic interaction among the magnetic crystalline grains and improve the crystalline orientation, and consequently the media noise of magnetic recording media will be reduced. When only the second additive element is added to Co without the first additive element, the inplane coercivity Hc is decreased, and thus the read output is liable to decrease. When the first additive element and the second additive element are added to Co at the same time as in the present invention, the inplane coercivity Hc is also increased, as shown in FIG. 16, and thus the media noise is lower than that in the case of only addition of the first additive element (Pt and Ir) to Co. Thus, a higher S/N ratio can be obtained as a result. The magnetic recording media shown in FIG. 16 has a magnetic layer of Co-15 at.% Cr-7 at.% Pt-3 at.% Si having a thickness of 65 nm, a Cr underlayer having a thickness of 350 nm and a C protective layer having a thickness of 40 nm. In this case, it is desirable that the magnetic layer has an oxygen concentration of 0.1 at.% or more. When the oxygen concentration of the magnetic layer exceeds 10 at.%, the oxidation considerably proceeds, and the saturation magnetization and the inplane coercivity are also lowered, resulting in a considerable decrease in the read output. Thus, a practically preferable oxygen concentration is 0.1 to 10 at.%. The oxygen concentration of the magnetic layer can be adjusted to a desired value in case of making the layer, for example, by sputtering while adjusting the oxygen partial pressure in the discharge gas atmosphere of Ar, etc.
In the foregoing, functions and the effective concentrations of the additive elements have been explained, and it is desirable that the sum total of the additive elements including the oxygen is 50 at.% at most, that is, Co as the main component that constitutes the balance has a concentration of at least 50 at.%.
The foregoing functions will be further explained below from the crystallographic viewpoint.
When a magnetic layer is formed on a nonmagnetic substrate through a nonmagnetic metallic underlayer by sputtering in an Ar gas containing 0.1 vol.% of oxygen, as explained before, an inplane coercivity Hc of 1,200 Oe or more can be obtained at a concentration of the first additive element (exemplified by Pt) of 1 to 35 at.%, as shown in FIG. 6. By adding Pt to Co, a Co-Pt ordered plase appears in the crystalline grains to suppress the movement of magnetic domain boundary. The inplane coercivity becomes maximum at a Pt concentration of 13 at.%. In connection to the appearance of Co-Pt ordered plase, the mechanism of the development of inplane coercivity is different between a magnetic layer having a Pt concentration of more than 13 at.% and a magnetic layer having a Pt concentration of less than 13 at.%, and particularly a dynamic magnetization reversal takes place smoothly at a Pt concentration of less than 13 at.%. Correspondingly, magnetic recording media having a Pt concentration of less than 13 at.% have particularly high overwrite characteristics and have a high efficiency of leakage recording in the track width direction, a high efficiency of erase and an effect-of broad margin for a position error at the read and write runs with a magnetic recording head. The effect is particularly remarkable at a Pt concentration of 9 at.% or less. Thus, a preferable Pt concentration is less than 13 at.%, more preferably, 9 at.% or less. With increasing Pt concentration, the saturation magnetization is slowly decreased. That is, there is such a tendency that the saturation magnetization will be decreased and the media noise will be relatively increased at a Pt concentration of 3 at.% or more, and, as shown in FIG. 9 the S/N ratio becomes particularly high at a Pt concentration of 1 to 3 at.%. Furthermore, Pt and Ir are expensive noble metals, and addition-of these metals in an unnecessarily large amount is not preferable with respect to cost. A practically more preferable concentration of Pt is 1 to 3 at.%, as described above. When the concentration of the first additive element such as Pt is more than 3 at.%, it is desirable to improve the overwrite characteristics by making a concentration of Pt and Ir less than 13 at.%, preferably.9 at.% or less, as described above, and also to improve the corrosion resistance by making a concentration of Co less than 75 at.% to considerably reduce the media noise, resulting in an increase in the S/N ratio. As the second additive element to be added to Co together with the first additive element such as Pt, a group of Ge and Si are particularly desirable besides Cr, Mo and W as shown in FIG. 6, and it is needless to say that other elements of the group, i.e. Ti, Zr, Hf, V, Nb and Ta are also effective. Particularly, in case of quaternary magnetic alloys containing the other elements of the group, oxides or hydroxides of these elements predominantly segregate on the surface or at the crystalline grain boundary to considerably improve the corrosion resistance, as compared with the ternary alloys, as shown in FIG. 12. Thus, the quaternary magnetic alloys are particularly preferable. FIG. 12 shows a relative magnetization Ms(t)/Ms(o) indicating a degree of deterioration due to corrosion on the ordinate and time (hr) of NaCl spray test at 400xc2x0 C. on the abscissa, and shows that the magnetic layers maintaining a relative magnetization of initial level for a longer time have a good corrosion resistance. Magnetic layers of Co-20 at.% Si-8 at.% Pt. Co-10 at.% Ge-8 at.% Pt and Co-8 at.% Pt (comparative) likewise formed have relative magnetizations of 0.85, 0.82 and 0.75, respectively, 4 hours after the NaCl spray test.
In the quaternary magnetic alloys of the present invention, Ti, Zr, Hf, V, Nb, Tal etc. segregate at the crystalline grain boundary and in the crystalline grains owing to the synergistic effect of Cr. Mo, etc. to reduce the magnetic interaction among the crystalline grains and improve the read and write characteristics, as described before, and thus the quaternary magnetic alloys are more desirable than the ternary magnetic alloys of CoPt containing one of Cr, Mo, W, Ge, Si, etc. In FIG. 12, only the Co-Cr-Pt-based magnetic alloys are exemplified, but Mo, W, Si, and Ge hold valid in place of Cr. A preferable concentration of at least one of these other elements of the group is 1 to 15 at.%.
FIG. 7 shows relationships between the Cr concentration and the inplane coercivity Hc when Cr is added to Co-Pt as typical of the second additive element, and it is desirable to add at least 1 at.%, particularly 3 at.% or more of Cr to Co-Pt, because the inplane coercivity exceeds 1,200 Oe thereby. Addition of more than 17 at.% of Cr is not desirable, because the saturation magnetization is deteriorated. Thus, an effective concentration of the second element is 1 to 17 at.%, except for Si, whose effective concentration is 1 to 40 at.%.
The nonmagnetic metallic underlayer of magnetic recording media shown in FIGS. 6 and 7 is composed of Cr, and it is needless to say that similar effects can be obtained with other than Cr, i.e. Mo, W, V, Nb or Ta or alloys containing these metals as the main component.
The concentration of oxygen in the magnetic layer will be described in more detail below.
When the present Co-based alloy containing the first and second additive elements further contains 0.1 to 10 at.% of oxygen, not only the component of inplane (10{overscore (1)}0) orientation (as will be hereinafter referred to xe2x80x9c(100) orientationxe2x80x9d) of hexagonal closed packed (hcp) structure, but also the component of perpendicular (0001) orientation (as will be-hereinafter referred to as xe2x80x9c(001) orientationxe2x80x9d) is increased even on a nonmagnetic metallic substrate of body-centered cubic (bcc) structure of Cr, Mo, W, etc. That is, from the crystallographic viewpoint, a ratio of 002 X-ray diffraction peak intensity to 100 X-ray diffraction peak intensity of Co-based alloy becomes more than 3 as shown in FIG. 13A and FIG. 13B, and from the magnetic viewpoint, a perpendicular anisotropy is given in addition to the basic inplane anisotropy, and the c-axis of Co-based alloy becomes substantially isotropic. As shown in FIG. 8, the inplace coercive squareness S* will be 0.85 or less, or further 0.8 or less with increasing thickness of the nonmagnetic metallic underlayer, because the second additive element such as Cr, Mo or W is liable to segregate not only at the crystalline boundary but also in the crystalline grains with the help of oxygen and/or the crystalline grains are made finer and/or the crystalline grains component that perpendicularly orient are increased. With an increase in the perpendicular anisotropy, the width of magnetic transition region becomes smaller, resulting in a decrease in the media noise. Thus, this is preferable. The oxygen concentrations of the magnetic layers of the present invention shown in FIGS. 6 and 7 are 1 and 1.5 at.%, respectively.
The fourth object of the present invention can be attained by a magnetic memory apparatus, which comprises a magnetic recording medium, a driving means for turning the magnetic recording medium, a magnetic head, a head access means, and a read and write means for the magnetic head, where the magnetic recording medium is composed of a magnetic recording medium for longitudinal recording, capable of attaining the first or second object or both objects of the present invention.
When the present magnetic recording media were subjected to read and write runs with a metal-in-gap (MIG) type or thin film head provided with a ferromagnetic metal such as Co-Nb-Zr, Fe-Al-Si, Ni-Fe, etc. at a position near the working gap, it was found that the read output was remarkably increased at an inplane coercivity Hc of 1,200 Oe or more in the circumferential direction of disk, as shown in FIG. 10. An inplane coercivity of 1,500 or more is more preferable, because the recording density can be further increased. When at least a portion of the magnetic pole is made of a ferromagnetic metal as mentioned above, the recording magnetic field can be intensified. Thus, use of a ferromagnetic metal is quite suitable for a magnetic recording medium with a high coercivity as in the present invention and can improve the read and write characteristics, particularly in a magnetic memory apparatus of larger capacity.